Artificial nail enhancements were first conceived in the mid 1950's as decorative shields for the finger nail (U.S. Pat. No. 2,816,555). They have since become commonplace in the consumer retail environment and are readily available in various forms from professional nail salons. Indeed, there are a number of variations to color the artificial nail enhancements from paint/pigments to metallic foils to thermochromic cholesteric liquid crystals (U.S. Pat. No. 4,920,991). This disclosure pertains to the use of bistable reflective electro-optic materials, such as bistable cholesteric liquid crystal materials, to form color changing artificial nails.
There are many reflective display technologies. However, many of them are not attractive for a color changing artificial nail application because of the many requirements: high reflectivity, wide color gamut, low cost, mechanical robustness, and thin conformable geometries in addition to low-power detached electronics. An old and well known bistable technology is the electrochromic display technology. This technology, however, has insufficient color gamut and, even though bistable, is not a low power option in that it is current driven requiring significant power from the battery to change the color. The bistable electrophoretic technology is used in black and white displays and covers but it is not yet developed for color. Further, it is not attractive for conformable color changing nail development as it would require high resolution patterning of the primary colors which adds considerably to the cost and limits the brightness to less than 33%. Electromechanical displays also suffer from this 33% reflectance limitation. Electrowetting lenses (U.S. Pat. No. 6,934,792) have been proposed; however, this scheme results in low reflectivity, the inability to mix colors, and suffers from objectionable layer thickness. Polarizer-based LCD technologies (e.g., Zenthal Displays, bistable STN) have been made bistable for reduced power but these use color filters to achieve color and as such can suffer from not being reflective enough for color changing artificial nail applications.
A potential reflective technology that might be considered for color changing artificial fingernails is the electrowetting or electrofluidic display technology. One major limitation of this technology for artificial color changing nails is that power must be applied continuously to display a particular color, hence; one cannot decouple the electronics which is a required due to space limitations. At this time the bistable cholesteric liquid crystal technology is the preferred technology and the one most sufficiently advanced for color changing artificial nail applications.
Cholesteric liquid crystalline (ChLC) materials are unique in their optical and electro-optic features. They are bistable (see U.S. Pat. Nos. 5,347,811 and 5,453,863, which are incorporated by reference) so that no power is required to maintain a selected color for the artificial nail. These materials possess a helical structure in which the liquid crystal (LC) director twists around a helical axis. The reflected light is circularly polarized with the same handedness as the helical structure of the LC. They can be tailored to Bragg reflect light at a pre-selected wavelength and bandwidth by controlling the pitch of the helical twist through the concentration of chiral dopants and the birefringence of the nematic host, respectively. If the incident light is not polarized, it will be decomposed into two circular polarized components with opposite handedness and one of the components reflected.
The cholesteric material is typically electrically switched to either one of two stable textures: planar or focal conic as described, for example, in the U.S. Pat. No. 5,453,863. One can electronically switch between the two states with a voltage pulse of different magnitude. In the planar texture, the director of the LC (direction of the long axis of the molecule) is uniformly parallel to the plane of the substrates across the cell but has a helical twist perpendicular to the plane of the substrates. It is the helical twist of the uniform planar texture that Bragg reflects light in a selected wavelength band. The focal conic texture contains defects that perturb the orientation of the liquid crystalline helices. In the typical focal conic texture, the defect density is high; thus the helical domain size becomes small and randomized in orientation such that it is just weakly scattering and does not reflect impinging light (i.e., it is essentially transparent to incident light). Once the defect structures are created, they are topologically stable and cannot be removed unless by some external force such as an electric field or melting the material out of the liquid crystalline phase to the isotropic phase. Thus, the focal conic texture remains stable and forward scatters light of all wavelengths into an absorbing (usually black) background. To achieve full color displays, individual layers of red, green, and blue reflecting cholesteric liquid crystal can be stacked on top of one another (e.g., U.S. Pat. Nos. 6,654,080 and 6,377,321). The intensity of the reflected colored light can be adjusted by the voltage pulse so that a vertical stack of the three primary colors, red, green and blue can be mixed to show any color; 4096 colors have been demonstrated.